Optical device and method of manufacturing the same

ABSTRACT

An optical device comprising a substrate, a porous layer laid on the substrate having a pore diameter smaller than the wavelength of light and a crystal layer laid on the porous layer showing a refractive index greater than that of the porous layer is presented. The optical device is manufactured by a method comprising a step of forming a porous layer having a pore diameter smaller than the wavelength of light on the surface of a substrate and a step of forming a crystal layer showing a refractive index greater than that of the porous layer on the porous layer. Since the porous layer is clad, light can be confined with ease.

TECHNICAL FIELD

This invention relates to an optical device and a method ofmanufacturing the same. More particularly, the present invention relatesto an optical device to be used for optical communications and inrelation with information processing apparatus that utilize light aswell as to a method of manufacturing the same and also to a sensoradapted to highly sensitively detect various pieces of information suchas bio information.

BACKGROUND ART

Optical devices of a genre referred to as photonic crystal have beenattracting attention in recent years. Yablonovitch: E. Yablonovitch“Phys. Rev. Lett.” Vol. 58, p. 2059, 1987 describes that the developmentof photonic crystals is centered on a technique of producing a periodicdistribution of refractive index by forming a periodic structure for anoptical material and effectively utilizing the behavior of light in thespecific refractive index distribution and a technique of effectivelyutilizing a phenomenon that light emitting modes can be controlled whena light emitting material is placed in a specific refractive indexdistribution. The potential of these techniques as applied to opticaldevices are being discussed.

Relating to the optical device techniques, so-called DFB lasers realizedby effectively utilizing a one-dimensional periodic structure forsemiconductor lasers have been put to actual use. Such optical devicesare produced by applying one-dimensional photonic crystal. At present,efforts are equally being paid for basic research activities in thefield of applying two-dimensional photonic crystal having atwo-dimensional structure of cylindrical holes that are periodicallyarranged in a plane to components of optical communication devices.Furthermore, three-dimensional periodic structures referred to asthree-dimensional photonic crystals are also known.

Of photonic crystals, two-dimensional photonic crystals are attractingparticular attention for several reasons including that they provide ahigh degree of freedom and hence can be made to operate in asophisticated way if compared with one-dimensional photonic crystals andthat, on the other hand, they can be prepared relatively easily ifcompared with three-dimensional photonic crystals by utilizing the knownsemiconductor processing techniques. Basic research activities are inprogress for the purpose of developing various devices usingtwo-dimensional photonic crystal. Such devices are highly promising forfinding practical applications.

Devices of different types using two-dimensional photonic crystal areobjects of current researches. They include micro-waveguide circuits,wavelength filters and micro-lasers to be used as optical communicationdevices. Kawakami et al., “Photonic Crystal Technology and itsApplications” (CMC Publishing, 2002), pp. 252, 257 and 258 describessuch devices.

As for the field of the biotechnology and the related industry that hasbeen growing remarkably in recent years, the applicant of the presentpatent application has proposed “a Micro-Sensor Using a Micro-ResonatorLaser” in Japanese Patent Application No. 2002-299153 as an applicationof two-dimensional photonic crystal to a highly integrated and highlysensitive biosensor chip.

Of two-dimensional photonic crystals that are objects of researches forthe purpose of actually developing devices, two-dimensional slab typephotonic crystals have been prepared most numerously. The slab typerefers to the one in which light is confined in a direction not showingany periodic structure by sandwiching a high refractive index core layerbetween low refractive index clad layers so that light is confined tothe high refractive index core layer for propagation.

The thickness of the slab, or the core layer, is related to theconditions on which an electromagnetic wave mode of light can exist inthe direction of the thickness. Particularly, in a case where only asingle mode can exist, the optical path length obtained by multiplyingthe slab thickness by the refractive index is approximately about a halfof the wavelength. Thus, the optical path length of a single round tripis approximately equal to the wavelength. In other words, this providesthe smallest thickness for allowing light that makes a single round tripto interfere with light that makes several round trips so as to raisethe intensity of light. In reality, the thickness is computed by takingthe propagation of light to the clad layer into consideration (seeKoshiba, “Optical Waveguide Analysis”, 1990, Asakura-Shoten)

SOI wafers formed by using an SiO₂ layer (BOX (buried oxide) layer) thatis formed on an Si substrate as clad and forming an Si layer (SOI(silicon on insulator) layer) thereon as core have been attractingattention in recent years as two-dimensional slab type photonic crystals(see Notomi, “Applied Physics”, Vol. 72, No. 7, 2003, “Photonic CrystalSlabs Using SOI Slabs”).

The use of such an Si type material provides advantages including (1)the currently available SOI wafer preparing techniques are alreadyfeasible so that the necessary precision level can be attained and (2)the currently available sophisticated Si process techniques can beapplied to forming a periodic pattern on an SOI layer to be used for acore layer.

Other areas of utilization of SOI wafers for two-dimensional (2D) slabtype optical devices include Si fine wire waveguides. As in the case oftwo-dimensional slab type photonic crystals, researches and developmentsare under way for confining light to micro-waveguides of 1 μm or lessand realizing curved waveguide devices with a small radius of curvatureby utilizing a large refractive index difference between Si and SiO₂(see, above-cited Kawakami's paper, p. 252).

However, when such an SOI wafer is used for a 2D slab type photoniccrystal or a fine wire waveguide, it is necessary to use relativelythick BOX layers typically having a thickness of 1 μm or more because ofthe requirements to be met for confining light. When light is confinedto a core layer, it can propagate into the clad layer as pointed outabove and, if the clad layer is thin, propagating light of theevanescent mode is coupled with light of the radiation mode directed tothe substrate to give rise to a radiation loss in a direction toward thesubstrate. The above cited Kawakami's paper, pp. 257 and 258, describesa calculated thickness necessary for the BOX layer when the allowableloss is −40 dB.

To prepare an SOI wafer comprising BOX layers having a thickness of 1 μmor more, a so-called bonding technique needs to be used. Such techniquesare described in Celler and Yasuda, “Status Quo of SOI Wafers for MEMS”,2002. 5, Electronic Technology and also in Iyer and Auberton-Herve,“SILICON WAFER BONDING TECHNOLOGY for VLSI and MEMS applications” (EMISPROCESSING—SERIES 1, ISBN 0 85296 0395, 2002, The Institution ofElectrical Engineers.

However, a process of bonding wafers inevitably includes a bonding stepalong with a seed wafer cutting step and a plurality of starting waferpreparation steps including special steps such as an H+ ion implantingstep and other complex steps. Consequently, the structure of the deviceto be prepared on such a wafer and the device preparation process arevery special if compared with ordinary Si wafers. Then, the prepared SOIwafer is very expensive so that the applications of such wafers arelimited only to semiconductor logic circuits such as CPUs having a highadded value that makes the use of such wafers economically feasible.

This invention is intended to dissolve the above identified problems ofthe background art and it is an object of the present invention toprovide a highly functional high precision optical device realized byusing two-dimensional slab type photonic crystal or a fine wirewaveguide having a porous layer as clad.

Another object of the present invention is to provide a method ofmanufacturing a highly functional high precision optical device having alarge area and realized by using two-dimensional slab type photoniccrystal or a fine wire waveguide at low cost.

DISCLOSURE OF THE INVENTION

In an aspect of the present invention, there is provided an opticaldevice comprising a substrate, a porous layer laid on the substratehaving a pore diameter smaller than the wavelength of light and acrystal layer laid on the porous layer showing a refractive indexgreater than that of the porous layer. Preferably, an optical deviceaccording to the invention is adapted to operate as an opticalresonator, a laser resonator in particular.

In another aspect of the invention, there is provided a method ofmanufacturing an optical device characterized by comprising a step offorming a porous layer having a pore diameter smaller than thewavelength of light on the surface of a substrate and a step of forminga crystal layer showing a refractive index greater than that of theporous layer on the porous layer.

In still another aspect of the invention, there is provided a sensorcomprising a porous layer having a pore diameter smaller than thewavelength of light, a crystal layer showing a refractive index greaterthan that of the porous layer and laid on the porous layer, a region inthe crystal layer showing a periodic distribution of refractive index, aflow channel for flowing fluid in the vicinity of the region and a meansfor irradiating light onto the region and detecting light emitted fromthe region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C are schematic views of a 2D slab type photoniccrystal device using porous Si for the clad layer of Example 1;

FIGS. 2A, 2B, 2C and 2D are schematic views of a 2D slab type photoniccrystal device using porous Si for the clad layer of Example 2;

FIG. 3 is a schematic illustration of a method of producing pores byanodization of Example 2;

FIG. 4 is a schematic illustration of another method of producing poresby anodization of Example 2;

FIGS. 5A, 5B and 5C are schematic views of an air bridge type 2D slabtype photonic crystal using porous Si of Example 7;

FIGS. 6A, 6B, 6C and 6D are schematic views of a 2D slab type photoniccrystal using a porous Ge clad and a GaAs core of Example 8, showing theconfiguration thereof;

FIGS. 7A, 7B and 7C are schematic views of a μTAS laser sensor systemusing a 2D photonic crystal of Example 9, showing the configurationthereof;

FIGS. 8A, 8B and 8C are schematic views of an Si fine wire waveguidedevice using porous Si for the clad layer of Example 11, showing theconfiguration thereof;

FIGS. 9A, 9B and 9C are schematic views of an Si fine wire waveguidedevice using porous Si for the clad layer of Example 12, showing theconfiguration thereof;

FIG. 10 is a schematic illustration of an alternative Si fine wirewaveguide device using porous Si for the clad layer of Example 12,showing the configuration thereof;

FIG. 11 is a schematic illustration of a surface oxidized Si fine wirewaveguide device using porous SiO₂ obtained by oxidation for the cladlayer of Example 13, showing the configuration thereof;

FIG. 12 is a schematic illustration of a laser sensor system usingphotonic crystal of Example 9, showing the configuration thereof;

FIGS. 13A, 13B, 13C, 13D, 13E, 13F and 13G are schematic views of aphotonic crystal of Example 3, showing the configuration thereof and amethod of preparing the same;

FIGS. 14A, 14B, 14C and 14D are schematic views of an air bridgephotonic crystal of Example 4, showing the configuration thereof and amethod of preparing the same;

FIGS. 15A, 15B, 15C, 15D, 15E and 15F are schematic views of an airbridge photonic crystal of Example 5, showing the configuration thereofand a method of preparing the same;

FIGS. 16A, 16B, 16C, 16D and 16E are schematic views of an activephotonic crystal of Example 6, showing the configuration thereof and amethod of preparing the same;

FIGS. 17A, 17B, 17C and 17D are schematic views of a photonic crystaldevice of Example 10, showing the configuration thereof and a method ofpreparing the same;

FIGS. 18A and 18B are schematic views of a flow-through type photoniccrystal sensor of Example 10, showing the configuration thereof and amethod of preparing the same;

FIGS. 19A and 19B are schematic views of a flow-through type photoniccrystal sensor of Example 10, showing an alternative configurationthereof and a method of preparing the same;

FIGS. 20A and 20B are schematic views of a flow-through type photoniccrystal sensor of Example 10, showing another alternative configurationthereof and a method of preparing the same;

FIGS. 21A, 21B, 21C, 21D and 21E are schematic views of an activephotonic crystal of Example 6, showing an alternative configurationthereof and a method of preparing the same;

FIGS. 22A, 22B, 29C and 22D are schematic views of an alternative deviceof Example 12, showing the configuration thereof; and

FIGS. 23A and 23B are schematic views of a method of preparing thedevice of Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION

An optical device according to the invention is a highly functional highprecision optical device realized by using two-dimensional slab typephotonic crystal or a fine wire waveguide having a porous layer as cladand can find applications in optical communications, informationprocessing apparatus adapted to use light and various highly sensitivesensors for detecting various pieces of information such as bioinformation.

While the present invention is described below by way of examples, thepresent invention is by no means limited to the embodiments described inthose examples, which may be modified and altered without departing fromthe scope of the invention in terms of sequence of operation and otheraspects.

While the present invention is described below in terms of materialsincluding Si, GaAs, Ge and GaP, the present invention is by no meanslimited thereto and compound semiconductors of the III-V type such asAlGaAs, InGaAs, InAs, GaInNAs, InGaP and InP and those of the II-VI typesuch as CdSe and Cds as well as combinations of epi-grown (epitaxiallygrown) materials and seed substrate materials showing lattice constantsand linear expansion coefficients that are close to each other can alsobe used for the purpose of the invention.

EXAMPLE 1

This example provides a 2D slab type photonic crystal device realized byusing a porous Si layer and an epi-Si layer respectively as clad andcore on an Si substrate. Now, a photonic crystal device of this examplewill be described by referring to FIGS. 1A through 1C.

Referring to FIGS. 1A through 1C, a porous Si layer 102 is formed on anSi substrate 101 to a thickness of about 1 μm and then a single crystalSi layer (epi-grown Si layer) 103 is formed thereon by epitaxial growthto a thickness of about 0.2 μm. A pattern 104 of cylindrical holes isformed in the single crystal Si layer 103 to show a triangle lattice.The cylindrical holes are formed to run through the single crystallayer. The cylindrical holes have a diameter and a cycle of arrangementthat are substantially equal to the wavelength of light so that only apart that shows a specific wavelength of the light being propagated inan intra-planar direction through the region of the single crystal layer103 where the pattern is formed is reflected or made to turn its movingdirection in a well-known manner.

A 2D slab type photonic crystal device according to the presentinvention is characterized in that the single crystal layer is held incontact with air along the top surface thereof and with the porous layer102 along the bottom surface thereof and, since the single crystal layershows a refractive index that is greater than the refractive index ofair and that of the porous layer 102, light is subjected to intra-planarconfinement. In other words, the single crystal layer operates as corelayer relative to the light being propagated and the porous layeroperates as clad layer. Thus, it is possible to prepare a waveguide thatgives rise to little loss of light or a resonance structure that shows ahigh Q value by using a 2D slab type photonic crystal device accordingto the invention.

The porous Si layer 102 is formed in such a way that the pore diameterof its pores is sufficiently smaller than the wavelength of light, e.g.,as small as 1/100 of the wavelength of light to be used with it. Sincethe pore diameter is sufficiently smaller than the wavelength of light,light is neither scattered nor deflected by any of the pores of theporous layer and hence only shows an average refractive index of light.The 2D slab type photonic crystal device of this example is adapted toan optical wavelength of 1.5 μm and the pore diameter is about 2 nm. Thedevice is prepared in such a way that the porosity of the device isabout 80% and the volumetric ratio of Si and air is about 2:8.

The average refractive index (to be referred to as effective refractiveindex hereinafter) of the porous Si layer can be approximatelydetermined by using formula (1) below and is about 1.5;n _(eff) =n _(air) ×x _(air) +n _(si) ×x _(si)   (1),where n_(eff) is the effective refractive index, n_(air) is therefractive index of the air filling the void pores, X_(air) is theporosity, n_(si) is the refractive index of Si and x_(si) is thevolumetric ratio of Si, which is equal to 1−x_(air). Since therefractive index n_(si) of silicon is 3.5,n _(eff)=1.0×0.8+3.5×0.2=1.5.

This value is substantially equal to the corresponding value of the SiO₂in the BOX layer of conventional SOI wafers. Since the difference Δnbetween it and the refractive index, or about 3.5, of the epi-Si layer,which is the core layer, is about 2, it is possible to strongly confinelight. The fact that it is possible to strongly confine light means thatlight can be made to locally exist within a small volume when the deviceis used as an optical waveguide or a resonator and hence it is possibleto realize a micro-optical device with an enhanced degree ofintegration.

The pattern 104 of periodic arrangement of airholes formed through thecrystal layer may show a square lattice, a honeycomb lattice or someother lattice pattern instead of the above described triangle lattice.

The periodic pattern 104 may include line defects 105 or point defects106 that are in fact not pores or point defects 107 whose diametersdiffer as a function of location in the crystal layer.

Of such defects, line defects can operate as optical waveguide thatconfines and propagates light along the lines while point defects canoperate as optical resonator that makes light exist locally in and/ornear them and confines it there. Thus, such defects can be designed andarranged freely depending on the application of the device. An opticalresonator can be formed by combining line defects and point defects inany of possible various different ways depending on the space mode oflight, introduction of light from and emission of light to the waveguideand other factors.

In the following description, the expression of periodic structure mayor may not include linear and/or point defects.

Thus, a 2D slab type photonic crystal device using a porous Si cladlayer that has a configuration as described above optically performsvery well and provides features that make the device adapted to a highdegree of integration like a 2D slab type photonic crystal device usingSOI particularly from the viewpoint of the above described effectiverefractive index. At the same time, it provides additional advantagesincluding that it can be realized at low cost by using a single materialto simplify the manufacturing process.

While the effective refractive index of the clad layer is 1.5 and thedifference of refractive index between it and the approximately 0.2 μmthick core layer is about 2 in this example, a smaller refractive indexmay be used for the purpose of the invention. If a smaller refractiveindex is used, the core layer is made to have a thickness greater than0.2 μm. Then, it is possible to reduce the porosity of the clad layer.

EXAMPLE 2

This example shows a method of preparing a 2D slab type photonic crystaldevice described in Example 1.

The method of preparing a photonic crystal device of this example willbe described below by referring to FIGS. 2A through 2D.

Firstly, a silicon layer 202 having a porous structure is formed on anSi substrate 201 as shown in FIG. 2A by anodization on the surface (FIG.2B).

It is well known that, when an electrochemical reaction is caused byusing an Si substrate as anode and flowing an electric current in ahydrofluoric acid solution, the pits (etch pits) formed on the surfaceare extended to give rise to void pores. As the void pores continue togrow at the front ends thereof, a porous layer is formed in the surfaceof the Si substrate to show a structure in which fine and oblong poresextend from the surface. The porous layer of a 2D slab type photoniccrystal device according to the invention is formed by utilizing thisphenomenon. The porous layer maintains the crystal bearing of theoriginal Si single crystal substrate and, as will be described ingreater detail hereinafter, it is possible to epitaxially grow thesingle crystal thereon.

Conditions for Anodization:

starting wafer: p⁺Si (100) 0.01 Ωcm

solution: a mixed solution of HF, C₂H₅OH and H₂O

anodization current: 150 mA/cm²

The anodization can be conducted by using an apparatus as shown in FIG.3. Referring to FIG. 3, an Si wafer (substrate) 301 is held in such away that its epi-Si layer is immersed in HF solution 302. The Si waferis held in position by a loser support body 305 and an upper supportbody 306 by way of an O-ring 303 and a Pt-made surface electrode 304. AnHF containing liquid tank is arranged in the upper support body 306 soas to communicate with the Si wafer 301 and is filled with HF solution302. A Pt-made mesh electrode 308 is arranged in the HF solution 302.The Pt-made surface electrode 304 and the Pt-made mesh electrode 308 areconnected respectively to anode 307 and cathode 309 so that carriers areinjected when a predetermined electric field is applied to the Si by wayof the HF solution 302 at the anode side and by way of the rear surfaceof the Si substrate at the anode side. However, it should be noted thatthe arrangement for anodization is not limited to the one described inthis example and any of various known appropriate arrangements mayalternatively be used for the purpose of the invention.

A plurality of wafers can be collectively processed on a batch basis bymeans of the apparatus of FIG. 4. The process of anodization becomesless expensive when wafers are treated on a batch basis.

The pore diameter, the density and the thickness of the porous siliconlayer can be controlled over a wide range by way of the composition ofthe solution for the anodization, the anodization current and theconductivity type and the electric conductivity of the substrate.Platinum or a metal coated with platinum is used for the electrodesbecause platinum can strongly withstand hydrofluoric acid. When aplurality of wafers are collectively subjected to anodization to formporous layers, the anodization solution that contacts the oppositesurfaces of the wafers operates as electrode as shown in FIG. 4. Then,the electrode can uniformly contact the wafers to enhance thecontrollability of the process of forming the porous layers. The porouslayer that is formed in this way maintains the crystal bearing of theoriginal single crystal substrate so that it is possible to epitaxiallygrow a uniform single crystal layer on top.

Then, an epi-Si layer 203 is formed on the porous Si layer 202 byepitaxial growth, typically using a chemical vapor deposition (CVD)method (FIG. 2C). It is important for this operation to be conducted ina hydrogen atmosphere because such an atmosphere seals the pores on thesurface of the porous layer in an accelerated manner and it is possibleto form a high-quality epitaxial layer on top (Yonehara et al.,Oyo-Buturi (Applied Physics) 2002 Comprehensive Reports, SeptemberIssue).

Conditions for Epitaxial Growth:

vapor phase growth temperature; 1,000° C.

gas: SiH₄/H₂

pressure: 700 Torr

Then, resist is applied onto the epi-Si layer 203 and a periodic pattern204 is formed by photolithography to form a mask. The single crystalepitaxial silicon layer is removed by etching by way of the mask to formcylindrical holes (FIG. 2D). At this time, the lower porous layer isleft to remain there so as to operate as clad layer. The silicon layeris etched on the following conditions.

Etching Conditions:

reactive ion etching

gas: Cl₂ type gas

Note that the silicon layer may be etched alternatively by using Brbased gas or by means of ECR plasma etching, ICP plasma etching or wetetching, if appropriate. It may be needless to say that resist may bereplaced by SiO₂ or some other appropriate substance for the mask of theetching operation.

When producing cylindrical holes in the epitaxial silicon single crystallayer grown on the porous silicon layer, it is desirable to use aselective etching technique that automatically stops the etchingoperation when the porous silicon layer becomes exposed.

It is known that porous silicon is generally highly reactive if comparedwith non-porous silicon because the surface area of porous silicon isvery large relative to the volume thereof (about hundreds of severalsquare meters per cubic centimeter). Remarkable examples of applicationof porous silicon include enhanced etching, enhanced oxidation and drugdelivery using the harmless bio solubility thereof.

Therefore, the porous silicon layer can hardly operate to stop theoperation of etching the epitaxial silicon layer. However, it can beused for that purpose by uniformly covering the lateral walls of thepores of the porous silicon with thin oxide film. This will be describedbelow.

Due to the mechanism of anodization of pores, all the pores are formedas the front ends thereof are extended. Therefore, the pores on thesurface communicate with the respective front ends before the epitaxialgrowth. Thus, as a result of thermal oxidation, oxygen is supplied tothe fine front ends to form a uniform oxide film coat. Note that thethermal oxidation needs to be conducted at a low temperature lower than500° C. At this temperature level, the volumetric ratio of the voidpores, or the porosity, does not change as a result of oxidation. If,however, the lateral walls of the void pores are subjected to thermaloxidation at a temperature higher than the above cited temperaturelevel, silicon atoms can move on the surfaces of the pores to deform andsometimes close the pores.

As an epitaxial layer is made to grow on the porous film in which thelateral walls of the void pores are oxidized, the porous layer can beused as etching stop layer when etching the epitaxial layer. Selectiveetching between silicon and silicon oxide is known in reactive ionetching (RIE). The epitaxial layer and the porous layer in which thelateral walls of the pores carry an oxide film coat can be selectivelyetched by modifying the conditions of the above selective etching. Theselectivity rises and, at the same time, the refractive index of theporous layer falls as thickness of the oxide film coat is increased.

As the pore surfaces of the porous layer is coated with oxide siliconfilm in a manner as described above, it is now possible to stop theetching at the porous layer when forming cylindrical holes in theepitaxial silicon layer so that a two-dimensional photonic crystal slabcan be prepared accurately at low cost in order to effectively confinelight. Then, it is possible to form various optical circuits.

For forming a chip of optical circuit devices and mounting it on any ofvarious systems, a cleaving/dicing technique for separating the devicesfrom the wafer as shown in FIGS. 23A and 23B can be used. With thistechnique, a partial wafer 2301 obtained by dicing a large wafer isfirstly cleaved in a direction substantially perpendicular to theoptical waveguide direction of the optical waveguide device pattern 2302to divide the partial wafer into a plurality of bar wafers 2303 (FIG.23A). Subsequently, the bar wafers 2303 are diced in a directionperpendicular to the longitudinal direction thereof to divide them intoindividual devices 2304 (FIG. 23B). The bar wafer is cleaved becausesignal input/output operations of each of the optical circuit devicesare conducted by way of end facets thereof and the smoother the endfacets, the higher the input/output efficiency, or the optical couplingefficiency, to improve the performance of the system. With a methodaccording to the invention, the core layer for optical waveguide, theclad layer and the substrate are all made to show a single crystalstructure so that flat end facts can easily be produced by cleavage ifthe intra-planar direction of the patterning operation is aligned withthe crystal bearing. Thus, a method according to the invention providesa remarkable advantage particularly from the viewpoint of massproduction if compared with a method of using a bonded SOI layer becausethe crystal bearing of the substrate differs from that of the SOI layer.

In this example, a cylindrical hole pattern showing a triangle latticeand including a defect waveguide or defect resonator similar to that ofExample 1 is formed as periodic pattern 204. The diameter of cylindricalholes is about 0.3 μm, which is equal to about ¼ of the operatingoptical communication band of 1.5 μm, and the pattern cycle is about 0.7μm.

Meanwhile, beside photolithography described above as patterningtechnique of this example, nano-imprinting that is less expensive, X-raylithography that provides a higher resolution, ion beam lithography, EBlithography or optical near field lithography may selectively be useddepending on the application of the device.

When oxidizing the porous layer, the porous layer may be subjected to anoxidation process once again after such a patterning operation, whereoxygen is introduced through a plurality of cylindrical holes and theoxidation process is continued by using the selectivity thereof untilall the porous Si becomes SiO₂ so that the produced porous SiO₂ may beused for the clad layer.

Thus, a 2D slab type photonic crystal device that does not require anybonding operation is prepared with the method of this example.

EXAMPLE 3

In this example, a method of preparing a 2D slab type photonic crystaldevice as described above in Example 1 that is different from that ofExample 2 is used. The method of this example differs from that ofExample 2 in that a crystal Si layer is formed on a porous layer not byepitaxial growth but by annealing a porous Si layer.

Now, the method of preparing a photonic crystal device of this examplewill be described below by referring to FIGS. 13A through 13G.

Firstly, an Si substrate 1301 illustrated in FIG. 13A is prepared andtwo porous silicon layers 1302 and 1303 are formed by means of atwo-stage anodization technique on the Si substrate 1301 as shown inFIGS. 13B and 13C. The two porous silicon layers show respectiveporosities that are different from each other. More specifically, theporosity of the upper porous Si layer 1302 is made to be lower than thatporosity of the lower porous Si layer 1303.

As in Example 2, a device showing a configuration as shown in FIG. 3 isused for the anodization.

The porosity of each of the porous layers is controlled by theanodization current used. In this example, the upper porous layer 1302that shows a small porosity is formed firstly by using a relativelysmall electric current in the step of FIG. 13B and then the electriccurrent is raised to form the lower porous layer 1303 that shows poreswith a large pore diameter in the step of FIG. 13C.

Conditions for Anodization:

starting wafer: p⁺Si (100) 0.01 Ωcm

solution: HF, C₂H₅OH and H₂O anodization current: 30 mA/cm² (upperlayer), 150 mA/cm² (lower layer)

Then, the lateral walls of the pores of the porous silicon are uniformlyand very thinly coated with oxide film (FIG. 13D). Due to the mechanismof anodization of pores, all the pores are formed by extention of thefront ends thereof. Therefore, the pores at the surface continuouslycommunicate with the respective front ends. Thus, oxygen is supplied tothe fine front ends to form, as a result of thermal oxidation, a uniformoxide film coat to both of upper and lower porus layers as schematicallyillustrated in FIG. 13D. Note that the thermal oxidation needs to beconducted at a low temperature lower than 500° C. so as not to reducethe porosity.

Conditions for Oxidation:

gas: O₂

temperature: 400° C.

Then, only the oxide film coat formed on the upper porous Si layer 1302is removed by an HF solution (FIG. 13E).

Thereafter, a hydrogen annealing operation is conducted on theconditions listed below.

Annealing Conditions:

gas: 100% H₂

temperature: 1,050° C.

Hydrogen annealing is a heating process that is conducted in a hydrogenatmosphere. As a result of hydrogen annealing, Si atoms in the upperporous Si layer 1302 not covered by the oxide film coat and its vicinitymoves to fill the void pores and form a continuous crystal layer 1304 asshown in FIG. 13F. At the same time, the surface is smoothed to anatomic level to produce a high quality optical waveguide core structurewhere, particularly, unwanted scattering of light by the surfaceroughness of the order of 1/10 of the wavelength of light is suppressed.Since the lower porous layer is not covered by an oxide film coat, voidpores are maintained there. Then, there is produced a light confiningstructure where the crystal layer 1304 formed by the annealing operatesas optical waveguide and the lower porous layer 1302 showing a loweffective refractive index operates as clad.

As described above, a crystal layer is formed not by using epitaxialgrowth but by using hydrogen annealing in this example. Thus, theprocess is simple and does not use SiH₄ and other similar gas.

Thereafter, as shown in FIG. 13G, a periodic pattern of cylindricalholes 1305 is formed on the crystal Si layer 1304 by removing thesilicon layer by etching, using a photolithography technique as inExample 2. The lower porous layer is left to remain there so as tooperate as clad layer. The silicon layer is removed by reactive ionetching using SF₆+CHF₃ gas.

It is possible to execute further oxidization process following the stepof FIG. 13G. In this process, an oxigen gas is introduced through thecylindrical holes 1305 to oxidize the lower porus Si layer 1303 almostwholely to SiO₂, which is preferable as a clad layer.

EXAMPLE 4

In this example, another method of preparing a 2D slab type photoniccrystal device is used. While two porous Si layers are formed as inExample 3, no oxide film coat is formed in the porous layers and the twoporous layers are annealed simultaneously in this example. The Sicrystal layer formed out of the upper porous layer by annealing is usedto operate as core for an optical waveguide and the cavities formed inthe lower porous layer at the same time are used to operate as lowerclad. As a result, an air bridge type 2D slab type photonic crystaldevice is formed in this example.

Now, the method of preparing a photonic crystal device of this examplewill be described below by referring to FIGS. 14A through 14D.

Firstly, two porous silicon layers 1302 and 1303 are formed by means ofa two-stage anodization technique on an Si substrate 1301 as shown inFIG. 14A (FIG. 14B). The two porous silicon layers show respectiveporosities that are different from each other. More specifically, theporosity of the upper porous Si layer 1302 is made to be lower than theporosity of the lower porous Si layer 1303.

As in Example 2, a device showing a configuration as shown in FIG. 3 isused for the anodization. The porosity of each of the porous layers iscontrolled by the anodization current used.

Conditions for Anodization:

starting wafer: p⁺Si (100) 0.01 Ωcm

solution: HF, C₂H₅OH and H₂O

anodization current: 30 mA/cm² (upper layer), 150 mA/cm² (lower layer)

Thereafter, a hydrogen annealing operation is conducted on theconditions listed below.

Annealing Conditions:

gas: 100% H₂

temperature: 1,050° C.

As a result of hydrogen annealing, Si atoms in the surface and itsvicinity of the upper porous Si layer 1302 moves to form a continuouscrystal layer 1304 (FIG. 14C). The surface is smoothed to an atomiclevel to produce a high quality optical waveguide core structure where,particularly, unwanted scattering of light by the surface roughness ofthe order of 1/10 of the wavelength of light is suppressed. Since thelower porous Si layer 1303 shows a high porosity and hence is short ofSi atoms, pores are coupled to one another to lower the surface energyso that consequently cavities 1401 are formed as shown in FIG. 14C. Thephenomenon that cavities are formed as a result of hydrogen annealing isdescribed in Japanese Patent Application Laid-Open No. 2000-144276. Thecavities 1401 operate effectively as lower clad for confining light inan optical waveguide device.

Then, a photolithography technique is applied to the crystal Si layer1304 on the cavities 1401 to selectively remove silicon layer and form aperiodic pattern of cylindrical holes (FIG. 14D). The etching operationfor selectively removing the silicon layer is typically conducted on thefollowing conditions.

Conditions for Processing the Silicon Layer:

etching: reactive ion etching

gas: SF₆+CHF₃ gas

In this example, a cylindrical hole pattern showing a triangle latticeand including a defect waveguide or defect resonator similar to that ofExample 1 is formed as periodic pattern 204. The diameter of cylindricalholes is about 0.3 μm, which is equal to about ¼ of the operatingoptical communication band of 1.5 μm, and the pattern cycle is about 0.7μm.

As a result of using an air bridge structure, the crystal layer isbrought to contact with air not only along the top surface but alsoalong the bottom surface thereof so as to show a refractive index thatis higher than that of the crystal layer when its bottom surface is heldin contact with a porous layer so that light is confined more reliably.Additionally, since the crystal layer is formed not by means ofepitaxial growth but by means of hydrogen annealing, the process issimple and does not use SiH₄ and other similar gas as in Example 3.

In the process of forming the upper porous Si layer 1302 as opticalwaveguide core layer 1304 by hydrogen annealing, any reduction in thefilm thickness is suppressed and the layer is formed reliably and stablywhen Si atoms are supplied in a vapor phase.

EXAMPLE 5

This example provides another example of preparing an air bridge type 2Dslab type photonic crystal device. The crystal Si layer is formed byepitaxial growth and the underlying porous Si layer is processed byannealing to produce cavities, which are used as lower clad.

Now, the method of preparing a photonic crystal device of this examplewill be described below by referring to FIGS. 15A through 15F.

Firstly, a silicon layer 1302 having a porous structure is formed on anSi substrate 1301 as shown in FIG. 15A by anodization on the surface(FIG. 15B).

As in Example 2, a device showing a configuration as shown in FIG. 3 isused for the anodization.

Conditions for Anodization:

starting wafer: p⁺Si (100) 0.01 Ωcm

solution: HF, C₂H₅OH and H₂O

anodization current: 150 mA/cm²

Then, the wafer is pre-baked in a hydrogen atmosphere to form acontinuous crystal thin film structure 1501 on the surface to seal thepores on the surface of the porous Si layer 1302 (FIG. 15C). It ispossible to supplement Si that is running short and reduce the crystaldefects of the crystal thin film structure 1501 by supplying Si in avapor phase.

Then, a crystal Si layer 1304 is added by epitaxial growth that startsfrom the surface of the crystal thin film structure 1501 (FIG. 15D).

Conditions for Epitaxial Growth:

vapor phase growth

temperature; 1,000° C.

gas: SiH₄/H₂

pressure: 700 Torr

Thereafter, a hydrogen annealing operation is conducted on the followingconditions.

Annealing Conditions:

gas: 100% H₂

temperature: 1,050° C.

As a result of the hydrogen annealing, the porous Si layer 1302 turnsinto cavities 1401 (FIG. 15E). The principle underlying the phenomenonof producing cavities as described above in Example 4 also applies here.In short, Si atoms moves from the porous Si layer and its vicinity andvoid pores are coupled to one another to produce cavities 1401.

As a result, an optical waveguide is formed and the crystal layer 1304operates as optical waveguide core while the cavities 1401 operate aslower clad layer there. Due to the hydrogen annealing, the wall surfacesof the cavities are smoothed to an atomic level to produce a highquality optical waveguide core structure in which unwanted scattering oflight by the surface roughness of the order of 1/10 of the wavelength oflight is suppressed along the top surfaces of the cavities, or theinterface of the cavity clad and the crystal layer optical waveguide.

Then, as in Example 4, an air bridge type 2D slab type photonic crystaldevice is prepared as a result of forming a periodic pattern in thecrystal Si layer 1304 that operates as optical waveguide core on thecavities 1401 (FIG. 15F).

In the process of sealing the void pores on the surface of the porouslayer, the thin film crystal layer may be formed in a simplified waywithout supplying Si in a vapor phase and adding an Si crystal layer bymeans of epitaxial growth.

EXAMPLE 6

This example provides an active photonic crystal structure obtained byintroducing an active medium into the clad layer of a 2D slab typephotonic crystal device using a porous Si layer as clad and an epi-Silayer as core and a method of preparing such a structure.

Now, the method of preparing a photonic crystal device of this examplewill be described below by referring to FIGS. 16A through 16E.

Firstly, a silicon layer 1302 having a porous structure is formed on anSi substrate 1301 as shown in FIG. 16A by anodization on the surface(FIG. 16B). A device showing a configuration as shown in FIG. 3 is usedfor the anodization.

Conditions for Anodization:

starting wafer: p⁺Si (100) 0.01 Ωcm

solution: HF, C₂H₅OH and H₂O

anodization current: 150 mA/cm²

Then, accelerated Er ions 1601 are implanted into the porous Si layer1302 to form an Er-doped region 1602 (FIG. 16C). If necessary, anannealing operation may additionally be conducted for the purpose ofactivation.

Then, a crystal Si layer 1304 is formed on the porous Si layer 1302 byepitaxial growth on the conditions listed below. It is important thatthis process is conducted in a hydrogen atmosphere so as to seal thepores on the surface of the porous layer and form a high qualityepitaxial layer thereon.

Conditions for Epitaxial Growth:

vapor phase growth

temperature; 1,000° C.

gas: SiH₄/H₂

pressure: 700 Torr

Thereafter, a photolithography technique is applied to the crystal Silayer 1304 to remove silicon layer and form a periodic pattern ofcylindrical holes. The lower porous layer is left to remain there so asto operate as clad layer. The etching operation for selectively removingthe silicon layer is typically conducted on the following conditions.

Conditions for Processing the Silicon Layer:

etching: reactive ion etching

gas: Cl₂ type gas

A cylindrical hole pattern showing a triangle lattice and including adefect waveguide or defect resonator is formed as periodic pattern 204.The diameter of cylindrical holes is about 0.3 μm, which is equal toabout ¼ of the gain wavelength zone of Er of 1 to 1.4 μm, or theoperating waveform, and the pattern cycle is about 0.7 μm. The defectresonator 1310 is aligned with the Er-doped region 1602. With thisarrangement, infrared rays transmitted through the Si photonic crystaland Er in the clad interact. Thus, it is possible to amplify infraredrays in the photonic crystal and produce laser oscillations by feedingback infrared rays to the photonic crystal defect resonator by means ofoptical switching using an nonlinear optical effect or by irradiating orintroducing excited light with a wavelength of or close to 1 μm to theEr-doped regions 1602.

With the above-described process of this example, an active 2D slab typephotonic crystal device containing an active medium in the clad layer isprepared.

While Er ions are used as active medium in this example, an organicfluorescent substance such as Alq3 or an inorganic fluorescent substancesuch as ZnS:Mn may alternatively be used. For example, an appropriatesolution may be prepared and the porous Si layer may be dipped into thesolution to adsorb such a substance into the void pores of the porouslayer, utilizing the capillary phenomenon.

Additionally, the active medium may be selected from crystal materialsGaAs, GaN, InGaN and AlInGaP. It is also possible to introduce such asubstance into the void pores of the porous layer and subject it tocrystal growth by means of a crystal growth system of MOCVD, CBE or MBE.

While the active medium is introduced into a part of the porous Si layerin this example, Er ions 2101 may alternatively irradiated onto theentire surface of the porous layer to produce a region 2102 that isdoped with the active medium over the entire surface as shown in FIG.21B. The remaining steps of the method of this example are same as thoseof FIGS. 16A through 16E.

EXAMPLE 7

In this example, an air bridge structure is formed by bringing anetching solution into contact with the porous Si layer of a 2D slab typephotonic crystal as prepared in Example 2 by way of the cylindricalthrough holes and partly removing the porous Si layer.

Now, the method of preparing a photonic crystal device of this examplewill be described below by referring to FIGS. 5A through 5C. Thestructure illustrated in FIG. 5A is same as that of the 2D slab photoniccrystal prepared in Example 2 and obtained as a result of the step ofFIG. 2D. More specifically, a porous Si layer 502 is formed on an Siseed substrate 501 and a single crystal Si layer 503 is formed thereonby epitaxial growth and subjected to a patterning operation to produce aphotonic crystal pattern 504 there.

In this example, the lower porous layer 502 of the structure is partlyremoved by etching through the cylindrical holes formed in the singlecrystal layer to produce a cavity, which is used as clad layer (FIG.5B).

Etching conditions:

solution: HF/H₂O

etching selectivity ratio:

-   -   crystal layer:porous layer 1:100,000

With the method of this example, it is possible to produce a cavityright under the cylindrical holes.

Then, hydrogen annealing is conducted on the following conditions. As aresult, the front surface, the lateral walls and the rear surface(cavity side of the air bridge) of the single crystal Si layer 505 thatcarries the pattern are smoothed.

Hydrogen Annealing Conditions:

gas: 100% H₂

temperature: 1,050° C.

The propagation loss is reduced when the smoothed photonic crystal isused for a waveguide, whereas a high Q value is obtained as a result ofsuppressing the loss when the smoothed photonic crystal is used for aresonator.

EXAMPLE 8

This example provides a method of preparing a 2D slab type photoniccrystal device, using a porous Ge layer as clad and an epi-GaAs layer ascore.

Now, the method of preparing a photonic crystal device of this examplewill be described below by referring to FIGS. 6A through 6D.

Firstly, a Ge layer 602 having a porous structure is formed on a Gesubstrate 601 as shown in FIG. 6A by anodization (FIG. 6B). A deviceshowing a configuration as shown in FIG. 3 may be used for theanodization. The process of anodization becomes less expensive when anumber of wafers are treated on a batch basis by means of a device asshown in FIG. 4.

Conditions for Anodization:

starting wafer: p⁺Ge(100) 0.01 Ωcm

solution: a mixed solution of HF, C₂H₅OH and H₂O

anodization current: 100 mA/cm²

Then, an epi-GaAs layer 603 is formed on the porous Ge layer 602 byepitaxial growth. Thereafter, a periodic pattern 604 is formed in theEpi-GaAs layer 603 by means of a photolithography technique.

In this example, a cylindrical hole pattern showing a triangle latticeand including a defect waveguide or defect resonator similar to that ofExample 1 is formed as periodic pattern 604. The diameter of cylindricalholes is about 0.3 μm, which is equal to about ¼ of the operatingoptical communication band of 1.5 μm, and the pattern cycle is about 0.7μm.

Meanwhile, beside photolithography described above as patterningtechnique of this example, nano-imprinting that is less expensive, X-raylithography that provides a higher resolution, ion beam lithography, EBlithography or optical near field lithography may selectively be useddepending on the application of the device.

Thus, a 2D slab type photonic crystal device that does not require anybonding operation is prepared with the method of this example.

Since GaAs, which is a direct transition type optical semiconductor, isused for the core layer, the device of this example can be used foremitting light by excitation of light or for a switching device thatutilizes the optical nonlinearity of the device. Thus, it is possible toproduce highly sophisticated devices by using the method of thisexample.

While GaAs is used in this example, any other crystal material mayalternatively be used without problem so long as the lattice constantand the linear expansion coefficient of the material are close to thoseof Ge.

EXAMPLE 9

Now, the ninth example of the present invention will be described byreferring to FIGS. 7A through 7C. FIGS. 7A through 7C illustrate a μTAS(micro-total analysis system) sensor system using a photonic crystalaccording to the invention.

FIG. 7A is a schematic illustration of the flow channel system of a μTASand photonic crystal laser sensors, showing their positionalrelationship. As shown in FIG. 7A, flow channels 702 are formed in aflow channel substrate 701 and liquid 703 that contains information tobe detected is made to flow there. As shown in FIG. 7A, the flow channelsystem may have an appropriate configuration so as to make it adapted toagitation, reaction or some other operation specific to μTAS as well asbranches, junctions and so on. It will be seen from the perspectiveillustration that photonic crystal laser sensors 704 are arrangedimmediately below the flow channels.

The photonic crystal lasers of this example are prepared by formingpoint defect type resonators, using 2D slab type photonic crystal, andarranging laser mediums at the point defects, which are excited by alight exciting means (not shown).

FIG. 12 schematically illustrates exemplary photonic crystal lasers. Asshown in FIG. 12, a porous Ge layer 1202 is formed on a Ge substrate1201 as clad layer and then a GaAs layer is formed thereon as core layerby epitaxial growth. Then, a multiple quantum well structure 1205 isformed thereon by using a ternary compound of AlGaAs as active layer1204, or a laser medium layer. Subsequently, the structure 1205 issubjected to a patterning operation to form a periodic cylindrical holepattern of photonic crystal that includes a defect resonator part 1206and then an air bridge structure is produced.

Of photonic crystal lasers having such a configuration, the thresholdvalues, the conditions of oscillation and the state of oscillation reactvery sensitively to the surrounding environment so that it can be usedas micro-laser sensor. The state of laser oscillation of the photoniccrystal laser sensors of this example changes very sensitively as afunction of the concentration of the substances contained in the fluidflowing through the fluid paths, the refractive index, the temperatureand the pressure of the fluid and other factors so that it is possibleto detect the changes in the fluid by detecting the state of the laserbeam output. As shown in FIG. 7C, the laser beam output is detected bythe lowermost light receiving layer 709 and the state of oscillation ofeach laser sensor is detected.

When the laser is excited by injecting an electric current, the state ofoscillation of the laser sensor can be observed by detecting the changesin the electric current that is being injected instead of detecting thelaser beam output.

FIG. 7C is a schematic cross sectional view of the sensor system of thisexample. As described above, flow channels 702 are formed in the flowchannel layer 701 and a cover layer 705 is formed thereon to close theflow channels 702 at the tops thereof. The flow channel 702 is alsoclosed at the bottoms thereof by a thin film 706 and photonic crystallasers 704 are arranged in contact with the thin film 706. The thin film706 has a thickness substantially equal to the oscillation wavelength ofthe photonic crystal lasers so that evanescent light from the photoniccrystal laser resonators reaches the fluid 703 in the fluid paths 702.At the same time, the designed loss of the laser resonators is made veryclose to the conditions to be met for oscillation. In other words, aslight change in the fluid can stop or trigger oscillation.

Thus, it is possible to produce a μTAS sensor system by using a photoniccrystal and a method of preparing the same according to the invention.

EXAMPLE 10

A 2D slab type photonic crystal is applied to a sensor in this example.Through holes are formed in the single crystal layer of a photoniccrystal device to show a periodic pattern and utilized as flow channels.

The sensor will be described by referring to FIGS. 17A through 17D, 20Aand 20B.

FIGS. 17A and 17B schematically illustrate the basic steps of the methodof preparing a sensor of this example. Firstly, a part of a surface ofan Si wafer 1301 that makes a rectangular region 1701 and an opticalwaveguide region (not shown) extending in an intra-planar direction issubjected to an operation of anodization to make it show pores (FIG.17A).

Then, the void pores exposed to the surface of the porous layer arepre-baked in an hydrogen atmosphere to seal them and subsequently acrystal Si layer 1702 is formed on the surface of the Si wafer byepitaxial growth (FIG. 17B).

Thereafter, a photonic crystal pattern 1305 and an optical waveguidepattern 1703 are formed in the crystal Si layer by patterning (FIG.17C).

Then, etchant is made to infiltrate through the plurality of cylindricalholes formed as as pattern in the photonic crystal in order to removethe porous Si in the rectangular region 1701 below the photonic crystalregion by selective wet etching and produce a flow channel 1704 (FIG.17D).

The optical device/flow channel structure 1801 prepared in this way isthen bonded to an upper flow channel structure 1802 made of PDMS(polydimethylsiloxane) and prepared separately as shown in FIG. 18A toproduce a sensor having a flow-through structure (FIG. 18B).

The transmission characteristics of the photonic crystal 1305 changedepending on the type and the properties of the object fluid that isguided by the flow channels 1803 and 1704 to flow through thecylindrical holes of the photonic crystal. It is possible to detect theobject substance by optical spectrum observation through the opticalwaveguide 1703.

Alternatively, through holes may be formed at an end of the flow channel1704 from the Si layer as shown in FIG. 19A. Then, through holes can beformed simultaneously with the patterning operation for the photoniccrystal and the optical waveguide. With this arrangement, it is possibleto form an inflow channel 1803 and an outflow channel 1903 in a samelayer as viewed in the multilayer direction to a great advantage forforming a system that comprises such a sensor and is connected to aso-called μ-TAS system.

Still alternatively, it is also possible to produce a flow throughstructure having cylindrical holes formed through the photonic crystal1305 and through holes formed from the rear surface of the Si substrate2001 of a photonic crystal device, in which a porous Si layer 2004 isformed on the entire surface of the substrate 2001 as shown in FIG. 20A.This arrangement provides a first advantage that the structure is verystrong because the thin film of the photonic crystal is supported by theporous layer and a second advantage that the flow through of the devicecan be controlled in various different ways to correspond to theproperties of the object fluid by controlling the porosity and the porediameter of the porous layer. For example, when detecting protein in asolution, the size of the void pores is made to of the order of the sizeof the object protein so as to prevent protein, or solute, from passingthrough the porous layer and encourage protein to adhere to the voidpores in the clad layer and the cylindrical holes in the photoniccrystal.

Thus, as described above, it is possible to prepare a flow through typesensor by using a photonic crystal device having a porous layeraccording to the invention.

While PDMS is used as material for forming the upper flow channelstructure in this example, it may be needless to say that some otherappropriate material such as Si, quartz or glass may alternatively beused.

EXAMPLE 11

This example provides an optical device, which is typically a fine wirewaveguide, formed by using porous silicon.

This example will be described below by referring to FIGS. 8A through8C. Referring to FIG. 8A through 8C, a porous silicon layer 802 isformed by anodization on a silicon substrate 801, which is a seed wafer,and a single crystal silicon layer 803 is formed further thereon byepitaxial growth as in Example 1.

A fine wire pattern is formed from the single crystal silicon layer 803to produce a waveguide by means of a photolithography technique ofapplying resist and conducting a patterning operation by means of anexposure system, which is followed by an etching operation. The poroussilicon layer 802 is subjected to selective etching. More specifically,the lateral walls of the pores of the porous silicon are coated by avery thin oxide film and the silicon epitaxial layer and the porouslayer, in which the lateral walls of the pores are covered by an oxidefilm coat, are subjected to selective etching, using a reactive ionetching (RIE) technique. The selectivity of the etching rises and, atthe same time, the refractive index of the porous layer falls when arelatively thick oxide film coat is used.

The fine wire surface and the lateral walls are smoothed by hydrogenannealing on the following conditions.

Annealing Conditions:

gas: 100% H₂

temperature: 1,050° C.

As a result of hydrogen annealing, the fine wire surface and the lateralwalls are smoothed to an atomic level. Particularly, the structure thatgenerates unwanted scattering Of light by the surface roughness of theorder of 1/10 of the wavelength of light is eliminated so that it ispossible to remarkably suppress the optical loss as an importantcharacteristic feature of fine wire waveguide. When an optical resonatoris produced by forming an annular fine wire waveguide (see Kawakami etal., ibid, p. 262), the Q value, which indicates an importantcharacteristic of the resonator, is raised to by turn improve thewavelength filtering characteristic and the laser oscillation thresholdvalue of the device realized by using the resonator.

Thus, a silicon fine wire waveguide whose surface is smoothed andoptical loss is suppressed is formed to use the low refractive indexporous silicon layer as lower clad.

While a photolithography technique is used for the patterning operationin this example, nano-imprinting that is less expensive, X-raylithography that provides a higher resolution, ion beam lithography, EBlithography or optical near field lithography may selectively be useddepending on the application of the device.

EXAMPLE 12

This example provides another optical device, which is also a fine wirewaveguide, formed by using porous silicon.

This example will be described below by referring to FIGS. 9A through9C. Referring to FIGS. 9A through 9C, a porous silicon layer 902 isformed by anodization on a silicon substrate 901, which is a seed wafer,and a single crystal silicon layer 903 is formed further thereon byepitaxial growth as in Example 1. A fine wire pattern is formed from theporous silicon layer and the single crystal silicon layer to produce awaveguide by means of a photolithography technique of applying resistand conducting a patterning operation by means of an exposure system,which is followed by an etching operation.

The fine wire surface and the lateral walls are smoothed by hydrogenannealing on the following conditions.

Annealing Conditions:

gas: 100% H₂

temperature: 1,050° C.

As a result of hydrogen annealing, the fine wire surface and the lateralwalls are smoothed to an atomic level. Particularly, the structure thatgenerates unwanted scattering of light by the surface roughness of theorder of 1/10 of the wavelength of light is eliminated so that it ispossible to remarkably suppress the optical loss as an importantcharacteristic feature of fine wire waveguide. When an optical resonatoris produced by forming an annular fine wire waveguide (see Kawakami etal., ibid, p. 262), the Q value, which indicates an importantcharacteristic of the resonator, is raised to by turn improve thewavelength filtering characteristic and the laser oscillation thresholdvalue of the device realized by using the resonator.

Thus, a silicon fine wire waveguide whose surface is smoothed andoptical loss is suppressed is formed to use the low refractive indexporous silicon layer as lower clad.

While a photolithography technique is used for the patterning operationin this example, nano-imprinting that is less expensive, X-raylithography that provides a higher resolution, ion beam lithography, EBlithography or optical near field lithography may selectively be useddepending on the application of the device.

It may be needless to say that a fine wire waveguide of this example canbe combined with an above described 2D slab type photonic crystal in ahybrid mode. For example, as schematically shown in FIG. 10, a resonatorhaving a minimum mode volume may be connected to a waveguide by means ofpoint defects and line defects of photonic crystal while the line defectwaveguide may be connected to a fine wire waveguide of this example andthe diameter of the fine wire waveguide may be gradually increased in anadiabatic manner to connect it to an external optical fiber. Variousother combinations may also be conceivable depending on the object andthe application.

An adiabatically mode-transferred waveguide can be realized moreappropriately by a method according to the invention. For example, asshown in FIGS. 22A through 22D, two porous crystal layers 1302 and 1301may be made to show different thicknesses by changing with time therespective anodization currents for different positions 2201, 2202 and2203 as viewed in an intra-planar direction (FIG. 22B). Particularly, itis possible to produce an inclination in a span between two positions2202 and 2203 as viewed in the intra-planar direction. Then, an opticalwaveguide core whose thickness changes in an adiabatic manner can beobtained from it by hydrogen annealing. As shown in FIG. 22C and FIG.22D, which is a corresponding schematic plan view, it is possible toproduce a waveguide that is also adiabatic in a direction perpendicularto the plane. It is also possible to change the ratio of the thicknessesof two porous layers by irradiating light showing a luminancedistribution including an inclination for intra-planar positions insteadof changing the anodization currents at intra-planar positions due tothe boosting effect of anodization that is attributable to photocarriers.

EXAMPLE 13

This example provides an optical device formed by using porous siliconand transforming the porous silicon layer into a porous SiO₂ layer byoxidation.

Now, this device will be described by referring to FIG. 11. FIG. 11 is aschematic illustration of the optical device of this example, showing amanufacturing process thereof. In FIG. 11, the upper two views areperspective views and the lower two views are lateral views. The lefttwo views of FIG. 11 illustrate the device having a porous silicon layer1102 formed on a silicon substrate 1101, which is a seed wafer, byanodization as in Example 7 and a single crystal silicon layer 1103formed by epitaxial growth. A fine wire pattern is formed from theporous silicon layer and the single crystal silicon layer to produce awaveguide by means of a photolithography technique of applying resistand conducting a patterning operation by means of an exposure system,which is followed by an etching operation.

The fine wire surface and the lateral walls are smoothed by hydrogenannealing as in Example 7.

In this example, the specimen is further oxidized on the followingconditions for oxidation.

Conditions for Oxidation:

gas: O₂/H₂

temperature: 1,050° C.

The right views in FIG. 11 show the fine wire waveguide obtained afterthe oxidation. While the non-porous crystal silicon layer and theunderlying porous silicon layer of the fine wire waveguide are oxidizedas a result of oxidation, the Si in the porous layer is oxidized at arate that is about a hundred times greater than the rate of oxidation ofthe non-porous crystal Si. Thus, it is possible to oxidize almost allthe porous silicon layer (1104 in FIG. 11) and only the surface of thenon-porous crystal silicon layer (1105 in FIG. 11) by regulating theduration of the oxidation process. As a result, the bottom, the lateralsurfaces and the top surface of the fine wire optical waveguide aresurrounded by a thermal oxidation layer showing a uniform refractiveindex and light is confined by an air layer and a porous SiO₂ layer thatfurther surround the waveguide with a sufficient thickness. Theinterface between the silicon and the SiO₂ of the fine wire waveguidethat is produced by the oxidation is smooth and effective forsuppressing unwanted light loss due to scattering because the silicon issubjected to hydrogen annealing in advance. Thus, an excellent opticalwaveguide is formed directly on a silicon substrate without using anexpensive SOI substrate.

A legend of the reference symbols in the drawings is shown below.

-   101: Si seed substrate-   102: porous Si layer-   103: epi-grown Si layer-   104: photonic crystal pattern-   105: line defect waveguide-   106: point defect resonator-   107: point defect resonator-   201: Si seed substrate-   202: porous Si layer-   203: epi-grown Si layer-   204: photonic crystal pattern-   301: Si substrate for preparing photonic crystal-   302: HF solution-   303: O-ring-   304: Pt-made surface electrode-   305: lower support body-   306: upper support body-   307: anode-   308: Pt-made mesh electrode-   309: cathode-   401: Si substrate for preparing photonic crystal-   402: piping for vacuum chuck-   403: wafer holder-   404: HF solution tank-   405: HF solution-   406: anode-   407: cathode-   501: Si seed substrate-   502: porous Si layer-   503: epi-grown Si layer-   504: photonic crystal pattern-   505: air bridge section-   506: Si plane surface after annealing-   601: Ge seed substrate-   602: porous Ge layer-   603: epi-grown GaAs layer-   604: photonic crystal pattern-   701: flow channel substrate-   702: flow channel-   703: fluid-   704: photonic crystal laser sensor-   705: cover layer-   706: thin film-   707: photonic crystal layer-   708: laser beam output-   709: light receiving layer-   801: Si seed substrate-   802: porous Si layer-   803: epi-grown Si fine wire waveguide-   901: Si seed substrate-   902: porous Si layer-   903: epi-grown Si fine wire waveguide-   1001: photonic crystal defect resonator-   1002: photonic crystal line defect waveguide-   1003: Si fine wire waveguide-   1004: tapered section of Si fine wire waveguide-   1005: external optical waveguide/optical fiber system-   1101: Si seed substrate-   1102: porous Si layer-   1103: epi-grown Si fine wire waveguide-   1104: porous SiO₂ layer (thermally oxidized SiO₂)-   1105: epi-grown Si fine wire waveguide with thermally oxidized SiO₂    film-   1201: Ge seed substrate-   1202: porous Ge layer-   1203: GaAs crystal layer-   1204: active layer-   1205: multiple quantum well-   1206: defect resonator part-   1301: Si substrate-   1302: upper porous Si layer-   1303: lower porous Si layer-   1304: optical waveguide core Si crystal layer-   1305: photonic crystal pattern-   1401: lower clad cavity-   1501: crystal thin film structure-   1601: Er ion-   1602: Er-doped region-   1701: porous Si region-   1702: epitaxial Si layer-   1703: optical waveguide-   1704: flow channel cavity-   1801: optical device/flow channel structure-   1802: upper flow channel structure-   1803: upper flow channel-   1901: optical device/flow channel structure-   1902: flow channel through pore-   1903: discharge flow channel-   2001: optical device/flow channel structure-   2002: lower flow channel structure-   2003: lower flow channel-   2004: porous Si layer-   2101: active medium-   2102: porous crystal layer including active medium-   2201: intra-planar position 1-   2202: intra-planar position 2-   2203: intra-planar position 3-   2301: partial wafer-   2302: optical waveguide device pattern-   2303: bar wafer-   2304: optical waveguide device

This application claims priority from Japanese Patent Application Nos.2003-305486 filed Aug. 28, 2003 and 2004-244686 filed Aug. 25, 2004,which are hereby incorporated by reference herein.

1. An optical device comprising a substrate, a porous layer laid on thesubstrate having a pore diameter smaller than the wavelength of lightand a crystal layer laid on the porous layer showing a refractive indexgreater than that of the porous layer.
 2. The device according to claim1, characterized in that said crystal layer forms a waveguide forpropagating light in an in-plane direction.
 3. The device according toclaim 1, characterized in that said crystal layer shows a periodicrefractive index distribution in the layer.
 4. The device according toclaim 1, characterized in that said crystal layer has line defects orpoint defects arranged in the periodic refractive index distribution inthe layer.
 5. The device according to claim 4, characterized in thatsaid crystal layer forms an optical waveguide along said line defects.6. The device according to claim 4, characterized in that said crystallayer forms an optical resonator for localizing light around said pointdefects.
 7. The device according to claim 6, characterized in that saidoptical resonator is a laser resonator.
 8. The device according to claim1, characterized in that said crystal layer shows a fine wire patternand forms a waveguide.
 9. The device according to claim 8, characterizedin that said crystal layer shows a fine wire pattern and forms awaveguide with said porous layer.
 10. The device according to claim 1,characterized in that said crystal layer includes an air bridgestructure formed between the crystal layer and the substrate that isdevoid of a porous layer.
 11. The device according to claim 1,characterized in that said crystal layer having a large refractive indexshows a thickness that varies depending on the position on saidsubstrate in an intra-planar direction.
 12. The device according toclaim 1, characterized in that an active medium is introduced into saidporous layer.
 13. The device according to claim 1, characterized in thatsaid porous layer is a porous silicon layer, a porous silicon layerhaving its surface coated with silicon oxide or a porous silicon oxidelayer and said crystal layer is a single crystal layer.
 14. The deviceaccording to claim 1, characterized in that said porous layer is aporous germanium layer and said crystal layer is a crystal GaAs layer.15. A method of manufacturing an optical device characterized bycomprising a step of forming a porous layer having a pore diametersmaller than the wavelength of light on the surface of a substrate and astep of forming a crystal layer showing a refractive index greater thanthat of the porous layer on the porous layer.
 16. The method accordingto claim 15, characterized in that said step of forming a porous layeris a step of forming a porous layer by anodization on the surface of thesubstrate.
 17. The method according to claim 15, characterized byfurther comprising a step of annealing the porous layer in a hydrogenatmosphere and forming a cavity in the porous layer.
 18. The methodaccording to claim 15, characterized in that said step of forming aporous layer is a step of forming two porous layers showing respectiveporosities that are different from each other by means of two-stageanodization.
 19. The method according to claim 18, characterized byfurther comprising a step of annealing the porous layer showing thelower porosity in a hydrogen atmosphere and forming a crystal layer. 20.The method according to claim 19, characterized in that an oxide filmcoat is formed on the lateral walls of the pores of the porous layershowing the higher porosity before said step of annealing and forming acrystal layer.
 21. The method according to claim 18, characterized byfurther comprising a step of annealing the porous layer showing thehigher porosity in a hydrogen atmosphere and forming a cavity.
 22. Themethod according to claim 15, characterized in that said step of forminga crystal layer is a step of forming a crystal layer by epitaxialgrowth.
 23. The method according to claim 22, characterized by furthercomprising a step of oxidizing the surface of the porous layer beforesaid step of forming a crystal layer by epitaxial growth.
 24. The methodaccording to claim 15, characterized by further comprising a step offorming through holes in said crystal layer.
 25. The method according toclaim 24, characterized by further comprising a step of etching andremoving the porous layer under the crystal layer by way of the throughholes.
 26. The method according to claim 25, characterized by furthercomprising a step of annealing the crystal layer in a hydrogenatmosphere and smoothing the front surface and the rear surface of thecrystal layer and the lateral walls of the through holes after said stepof etching and removing said porous layer.
 27. The method according toclaim 15, characterized by further comprising a step of patterning thecrystal layer and etching and removing it to form a waveguide.
 28. Themethod according to claim 27, characterized by further comprising a stepof forming an oxide film coat on the lateral walls of the pores of theporous layer before said step of forming a waveguide.
 29. The methodaccording to claim 15, characterized by further comprising a step ofpatterning the crystal layer and the porous layer and etching andremoving them to form a waveguide.
 30. The method according to claim 29,characterized by further comprising a step of oxidizing the porous layerunder the crystal layer after said step of forming a waveguide.
 31. Themethod according to claim 30, characterized by further comprising a stepof annealing the waveguide in a hydrogen atmosphere and smoothing thesurface and the lateral walls of the waveguide after said step offorming a waveguide.
 32. The method according to claim 15, characterizedby further comprising a step of introducing an active medium into theporous layer after forming said porous layer.
 33. The method accordingto claim 15, characterized by further comprising a step of cleavingalong the crystal surface bearing of the substrate.
 34. A sensorcharacterized by comprising a porous layer having a pore diametersmaller than the wavelength of light, a crystal layer showing arefractive index greater than that of the porous layer and laid on theporous layer, a region in the crystal layer showing a periodicdistribution of refractive index, a flow channel for flowing fluid inthe vicinity of the region and means for irradiating light onto theregion and detecting light emitted from the region.
 35. The sensoraccording to claim 34, characterized in that said region showing adistribution of refractive index is located outside said flow channeland arranged in such a way that evanescent waves of light propagatingthrough the crystal layer in said region reaches the flow channel. 36.The sensor according to claim 34, characterized in that said regionshowing a distribution of refractive index is formed by through holesarranged in said substrate to form a periodic pattern and the throughholes take part of said flow channel.